The objectives of this work are to correlate the structure of RNA tumor virus genomes with their function in replication and in induction of disease. The AKR murine leukemia viruses and Rous sarcoma viruses will be used. SPECIFIC AIMS: A) Detailed comparison of the genome structures of the AKR viruses. This will include a study of the leukemogenic and non-leukemogenic ecotropic, xenotropic and polytropic viruses derived from the AKR mouse. B) Determination of the complete nucleotide sequence of an ecotropic and polytropic AKR virus. Techniques of RNA and DNA sequencing will be applid to this problem. C) Determination of the sequence of the 5' and 3' terminal regions of Rous sarcoma virus Prague B. D) Isolation, characterization and partial sequence determination of messenger RNA species of an AKR virus and of the 3' terminal Rous sarcoma virus messenger RNA's.